/*
** $Id: lgc.c,v 2.215 2016/12/22 13:08:50 roberto Exp $
** Garbage Collector
** See Copyright Notice in lua.h
*/

#define lgc_c
#define LUA_CORE

#include "lprefix.h"


#include <string.h>

#include "lua.h"

#include "ldebug.h"
#include "ldo.h"
#include "lfunc.h"
#include "lgc.h"
#include "lmem.h"
#include "lobject.h"
#include "lstate.h"
#include "lstring.h"
#include "ltable.h"
#include "ltm.h"


/*
** internal state for collector while inside the atomic phase. The
** collector should never be in this state while running regular code.
*/
#define GCSinsideatomic     (GCSpause + 1)

/*
** cost of sweeping one element (the size of a small object divided
** by some adjust for the sweep speed)
*/
#define GCSWEEPCOST ((sizeof(TString) + 4) / 4)

/* maximum number of elements to sweep in each single step */
#define GCSWEEPMAX  (cast_int((GCSTEPSIZE / GCSWEEPCOST) / 4))

/* cost of calling one finalizer */
#define GCFINALIZECOST  GCSWEEPCOST


/*
** macro to adjust 'stepmul': 'stepmul' is actually used like
** 'stepmul / STEPMULADJ' (value chosen by tests)
*/
#define STEPMULADJ      200


/*
** macro to adjust 'pause': 'pause' is actually used like
** 'pause / PAUSEADJ' (value chosen by tests)
*/
#define PAUSEADJ        100


/*
** 'makewhite' erases all color bits then sets only the current white
** bit
*/
#define maskcolors  (~(bitmask(BLACKBIT) | WHITEBITS))
#define makewhite(g,x)  \
 (x->marked = cast_byte((x->marked & maskcolors) | luaC_white(g)))

#define white2gray(x)   resetbits(x->marked, WHITEBITS)
#define black2gray(x)   resetbit(x->marked, BLACKBIT)


#define valiswhite(x)   (iscollectable(x) && iswhite(gcvalue(x)))

#define checkdeadkey(n) lua_assert(!ttisdeadkey(gkey(n)) || ttisnil(gval(n)))


#define checkconsistency(obj)  \
  lua_longassert(!iscollectable(obj) || righttt(obj))


#define markvalue(g,o) { checkconsistency(o); \
  if (valiswhite(o)) reallymarkobject(g,gcvalue(o)); }

#define markobject(g,t) { if (iswhite(t)) reallymarkobject(g, obj2gco(t)); }

/*
** mark an object that can be NULL (either because it is really optional,
** or it was stripped as debug info, or inside an uncompleted structure)
*/
#define markobjectN(g,t)    { if (t) markobject(g,t); }

static void reallymarkobject( global_State* g, GCObject* o );


/*
** {======================================================
** Generic functions
** =======================================================
*/


/*
** one after last element in a hash array
*/
#define gnodelast(h)    gnode(h, cast(size_t, sizenode(h)))


/*
** link collectable object 'o' into list pointed by 'p'
*/
#define linkgclist(o,p) ((o)->gclist = (p), (p) = obj2gco(o))


/*
** If key is not marked, mark its entry as dead. This allows key to be
** collected, but keeps its entry in the table.  A dead node is needed
** when Lua looks up for a key (it may be part of a chain) and when
** traversing a weak table (key might be removed from the table during
** traversal). Other places never manipulate dead keys, because its
** associated nil value is enough to signal that the entry is logically
** empty.
*/
static void removeentry( Node* n ) {
    lua_assert( ttisnil( gval( n ) ) );

    if ( valiswhite( gkey( n ) ) ) {
        setdeadvalue( wgkey( n ) );    /* unused and unmarked key; remove it */
    }
}


/*
** tells whether a key or value can be cleared from a weak
** table. Non-collectable objects are never removed from weak
** tables. Strings behave as 'values', so are never removed too. for
** other objects: if really collected, cannot keep them; for objects
** being finalized, keep them in keys, but not in values
*/
static int iscleared( global_State* g, const TValue* o ) {
    if ( !iscollectable( o ) ) {
        return 0;
    }
    else if ( ttisstring( o ) ) {
        markobject( g, tsvalue( o ) ); /* strings are 'values', so are never weak */
        return 0;
    }
    else {
        return iswhite( gcvalue( o ) );
    }
}


/*
** barrier that moves collector forward, that is, mark the white object
** being pointed by a black object. (If in sweep phase, clear the black
** object to white [sweep it] to avoid other barrier calls for this
** same object.)
*/
void luaC_barrier_( lua_State* L, GCObject* o, GCObject* v ) {
    global_State* g = G( L );
    lua_assert( isblack( o ) && iswhite( v ) && !isdead( g, v ) &&
                !isdead( g, o ) );

    if ( keepinvariant( g ) ) { /* must keep invariant? */
        reallymarkobject( g, v );    /* restore invariant */
    }
    else {  /* sweep phase */
        lua_assert( issweepphase( g ) );
        makewhite( g, o ); /* mark main obj. as white to avoid other barriers */
    }
}


/*
** barrier that moves collector backward, that is, mark the black object
** pointing to a white object as gray again.
*/
void luaC_barrierback_( lua_State* L, Table* t ) {
    global_State* g = G( L );
    lua_assert( isblack( t ) && !isdead( g, t ) );
    black2gray( t ); /* make table gray (again) */
    linkgclist( t, g->grayagain );
}


/*
** barrier for assignments to closed upvalues. Because upvalues are
** shared among closures, it is impossible to know the color of all
** closures pointing to it. So, we assume that the object being assigned
** must be marked.
*/
void luaC_upvalbarrier_( lua_State* L, UpVal* uv ) {
    global_State* g = G( L );
    GCObject* o = gcvalue( uv->v );
    lua_assert( !upisopen( uv ) ); /* ensured by macro luaC_upvalbarrier */

    if ( keepinvariant( g ) ) {
        markobject( g, o );
    }
}


void luaC_fix( lua_State* L, GCObject* o ) {
    global_State* g = G( L );
    lua_assert( g->allgc == o ); /* object must be 1st in 'allgc' list! */
    white2gray( o ); /* they will be gray forever */
    g->allgc = o->next;  /* remove object from 'allgc' list */
    o->next = g->fixedgc;  /* link it to 'fixedgc' list */
    g->fixedgc = o;
}


/*
** create a new collectable object (with given type and size) and link
** it to 'allgc' list.
*/
GCObject* luaC_newobj( lua_State* L, int tt, size_t sz ) {
    global_State* g = G( L );
    GCObject* o = cast( GCObject*, luaM_newobject( L, novariant( tt ), sz ) );
    o->marked = luaC_white( g );
    o->tt = tt;
    o->next = g->allgc;
    g->allgc = o;
    return o;
}

/* }====================================================== */



/*
** {======================================================
** Mark functions
** =======================================================
*/


/*
** mark an object. Userdata, strings, and closed upvalues are visited
** and turned black here. Other objects are marked gray and added
** to appropriate list to be visited (and turned black) later. (Open
** upvalues are already linked in 'headuv' list.)
*/
static void reallymarkobject( global_State* g, GCObject* o ) {
reentry:
    white2gray( o );

    switch ( o->tt ) {
    case LUA_TSHRSTR: {
        gray2black( o );
        g->GCmemtrav += sizelstring( gco2ts( o )->shrlen );
        break;
    }

    case LUA_TLNGSTR: {
        gray2black( o );
        g->GCmemtrav += sizelstring( gco2ts( o )->u.lnglen );
        break;
    }

    case LUA_TUSERDATA: {
        TValue uvalue;
        markobjectN( g, gco2u( o )->metatable ); /* mark its metatable */
        gray2black( o );
        g->GCmemtrav += sizeudata( gco2u( o ) );
        getuservalue( g->mainthread, gco2u( o ), &uvalue );

        if ( valiswhite( &uvalue ) ) { /* markvalue(g, &uvalue); */
            o = gcvalue( &uvalue );
            goto reentry;
        }

        break;
    }

    case LUA_TLCL: {
        linkgclist( gco2lcl( o ), g->gray );
        break;
    }

    case LUA_TCCL: {
        linkgclist( gco2ccl( o ), g->gray );
        break;
    }

    case LUA_TTABLE: {
        linkgclist( gco2t( o ), g->gray );
        break;
    }

    case LUA_TTHREAD: {
        linkgclist( gco2th( o ), g->gray );
        break;
    }

    case LUA_TPROTO: {
        linkgclist( gco2p( o ), g->gray );
        break;
    }

    default:
        lua_assert( 0 );
        break;
    }
}


/*
** mark metamethods for basic types
*/
static void markmt( global_State* g ) {
    int i;

    for ( i = 0; i < LUA_NUMTAGS; i++ ) {
        markobjectN( g, g->mt[i] );
    }
}


/*
** mark all objects in list of being-finalized
*/
static void markbeingfnz( global_State* g ) {
    GCObject* o;

    for ( o = g->tobefnz; o != NULL; o = o->next ) {
        markobject( g, o );
    }
}


/*
** Mark all values stored in marked open upvalues from non-marked threads.
** (Values from marked threads were already marked when traversing the
** thread.) Remove from the list threads that no longer have upvalues and
** not-marked threads.
*/
static void remarkupvals( global_State* g ) {
    lua_State* thread;
    lua_State** p = &g->twups;

    while ( ( thread = *p ) != NULL ) {
        lua_assert( !isblack( thread ) ); /* threads are never black */

        if ( isgray( thread ) && thread->openupval != NULL ) {
            p = &thread->twups;    /* keep marked thread with upvalues in the list */
        }
        else {  /* thread is not marked or without upvalues */
            UpVal* uv;
            *p = thread->twups;  /* remove thread from the list */
            thread->twups = thread;  /* mark that it is out of list */

            for ( uv = thread->openupval; uv != NULL; uv = uv->u.open.next ) {
                if ( uv->u.open.touched ) {
                    markvalue( g, uv->v ); /* remark upvalue's value */
                    uv->u.open.touched = 0;
                }
            }
        }
    }
}


/*
** mark root set and reset all gray lists, to start a new collection
*/
static void restartcollection( global_State* g ) {
    g->gray = g->grayagain = NULL;
    g->weak = g->allweak = g->ephemeron = NULL;
    markobject( g, g->mainthread );
    markvalue( g, &g->l_registry );
    markmt( g );
    markbeingfnz( g ); /* mark any finalizing object left from previous cycle */
}

/* }====================================================== */


/*
** {======================================================
** Traverse functions
** =======================================================
*/

/*
** Traverse a table with weak values and link it to proper list. During
** propagate phase, keep it in 'grayagain' list, to be revisited in the
** atomic phase. In the atomic phase, if table has any white value,
** put it in 'weak' list, to be cleared.
*/
static void traverseweakvalue( global_State* g, Table* h ) {
    Node* n, *limit = gnodelast( h );
    /* if there is array part, assume it may have white values (it is not
       worth traversing it now just to check) */
    int hasclears = ( h->sizearray > 0 );

    for ( n = gnode( h, 0 ); n < limit; n++ ) { /* traverse hash part */
        checkdeadkey( n );

        if ( ttisnil( gval( n ) ) ) { /* entry is empty? */
            removeentry( n );    /* remove it */
        }
        else {
            lua_assert( !ttisnil( gkey( n ) ) );
            markvalue( g, gkey( n ) ); /* mark key */

            if ( !hasclears && iscleared( g, gval( n ) ) ) { /* is there a white value? */
                hasclears = 1;    /* table will have to be cleared */
            }
        }
    }

    if ( g->gcstate == GCSpropagate ) {
        linkgclist( h, g->grayagain );    /* must retraverse it in atomic phase */
    }
    else if ( hasclears ) {
        linkgclist( h, g->weak );    /* has to be cleared later */
    }
}


/*
** Traverse an ephemeron table and link it to proper list. Returns true
** iff any object was marked during this traversal (which implies that
** convergence has to continue). During propagation phase, keep table
** in 'grayagain' list, to be visited again in the atomic phase. In
** the atomic phase, if table has any white->white entry, it has to
** be revisited during ephemeron convergence (as that key may turn
** black). Otherwise, if it has any white key, table has to be cleared
** (in the atomic phase).
*/
static int traverseephemeron( global_State* g, Table* h ) {
    int marked = 0;  /* true if an object is marked in this traversal */
    int hasclears = 0;  /* true if table has white keys */
    int hasww = 0;  /* true if table has entry "white-key -> white-value" */
    Node* n, *limit = gnodelast( h );
    unsigned int i;

    /* traverse array part */
    for ( i = 0; i < h->sizearray; i++ ) {
        if ( valiswhite( &h->array[i] ) ) {
            marked = 1;
            reallymarkobject( g, gcvalue( &h->array[i] ) );
        }
    }

    /* traverse hash part */
    for ( n = gnode( h, 0 ); n < limit; n++ ) {
        checkdeadkey( n );

        if ( ttisnil( gval( n ) ) ) { /* entry is empty? */
            removeentry( n );    /* remove it */
        }
        else if ( iscleared( g, gkey( n ) ) ) { /* key is not marked (yet)? */
            hasclears = 1;  /* table must be cleared */

            if ( valiswhite( gval( n ) ) ) { /* value not marked yet? */
                hasww = 1;    /* white-white entry */
            }
        }
        else if ( valiswhite( gval( n ) ) ) {   /* value not marked yet? */
            marked = 1;
            reallymarkobject( g, gcvalue( gval( n ) ) ); /* mark it now */
        }
    }

    /* link table into proper list */
    if ( g->gcstate == GCSpropagate ) {
        linkgclist( h, g->grayagain );    /* must retraverse it in atomic phase */
    }
    else if ( hasww ) { /* table has white->white entries? */
        linkgclist( h, g->ephemeron );    /* have to propagate again */
    }
    else if ( hasclears ) { /* table has white keys? */
        linkgclist( h, g->allweak );    /* may have to clean white keys */
    }

    return marked;
}


static void traversestrongtable( global_State* g, Table* h ) {
    Node* n, *limit = gnodelast( h );
    unsigned int i;

    for ( i = 0; i < h->sizearray; i++ ) { /* traverse array part */
        markvalue( g, &h->array[i] );
    }

    for ( n = gnode( h, 0 ); n < limit; n++ ) { /* traverse hash part */
        checkdeadkey( n );

        if ( ttisnil( gval( n ) ) ) { /* entry is empty? */
            removeentry( n );    /* remove it */
        }
        else {
            lua_assert( !ttisnil( gkey( n ) ) );
            markvalue( g, gkey( n ) ); /* mark key */
            markvalue( g, gval( n ) ); /* mark value */
        }
    }
}


static lu_mem traversetable( global_State* g, Table* h ) {
    const char* weakkey, *weakvalue;
    const TValue* mode = gfasttm( g, h->metatable, TM_MODE );
    markobjectN( g, h->metatable );

    if ( mode && ttisstring( mode ) && /* is there a weak mode? */
            ( ( weakkey = strchr( svalue( mode ), 'k' ) ),
              ( weakvalue = strchr( svalue( mode ), 'v' ) ),
              ( weakkey || weakvalue ) ) ) { /* is really weak? */
        black2gray( h ); /* keep table gray */

        if ( !weakkey ) { /* strong keys? */
            traverseweakvalue( g, h );
        }
        else if ( !weakvalue ) { /* strong values? */
            traverseephemeron( g, h );
        }
        else { /* all weak */
            linkgclist( h, g->allweak );    /* nothing to traverse now */
        }
    }
    else { /* not weak */
        traversestrongtable( g, h );
    }

    return sizeof( Table ) + sizeof( TValue ) * h->sizearray +
           sizeof( Node ) * cast( size_t, allocsizenode( h ) );
}


/*
** Traverse a prototype. (While a prototype is being build, its
** arrays can be larger than needed; the extra slots are filled with
** NULL, so the use of 'markobjectN')
*/
static int traverseproto( global_State* g, Proto* f ) {
    int i;

    if ( f->cache && iswhite( f->cache ) ) {
        f->cache = NULL;    /* allow cache to be collected */
    }

    markobjectN( g, f->source );

    for ( i = 0; i < f->sizek; i++ ) { /* mark literals */
        markvalue( g, &f->k[i] );
    }

    for ( i = 0; i < f->sizeupvalues; i++ ) { /* mark upvalue names */
        markobjectN( g, f->upvalues[i].name );
    }

    for ( i = 0; i < f->sizep; i++ ) { /* mark nested protos */
        markobjectN( g, f->p[i] );
    }

    for ( i = 0; i < f->sizelocvars; i++ ) { /* mark local-variable names */
        markobjectN( g, f->locvars[i].varname );
    }

    return sizeof( Proto ) + sizeof( Instruction ) * f->sizecode +
           sizeof( Proto* ) * f->sizep +
           sizeof( TValue ) * f->sizek +
           sizeof( int ) * f->sizelineinfo +
           sizeof( LocVar ) * f->sizelocvars +
           sizeof( Upvaldesc ) * f->sizeupvalues;
}


static lu_mem traverseCclosure( global_State* g, CClosure* cl ) {
    int i;

    for ( i = 0; i < cl->nupvalues; i++ ) { /* mark its upvalues */
        markvalue( g, &cl->upvalue[i] );
    }

    return sizeCclosure( cl->nupvalues );
}

/*
** open upvalues point to values in a thread, so those values should
** be marked when the thread is traversed except in the atomic phase
** (because then the value cannot be changed by the thread and the
** thread may not be traversed again)
*/
static lu_mem traverseLclosure( global_State* g, LClosure* cl ) {
    int i;
    markobjectN( g, cl->p ); /* mark its prototype */

    for ( i = 0; i < cl->nupvalues; i++ ) { /* mark its upvalues */
        UpVal* uv = cl->upvals[i];

        if ( uv != NULL ) {
            if ( upisopen( uv ) && g->gcstate != GCSinsideatomic ) {
                uv->u.open.touched = 1;    /* can be marked in 'remarkupvals' */
            }
            else {
                markvalue( g, uv->v );
            }
        }
    }

    return sizeLclosure( cl->nupvalues );
}


static lu_mem traversethread( global_State* g, lua_State* th ) {
    StkId o = th->stack;

    if ( o == NULL ) {
        return 1;    /* stack not completely built yet */
    }

    lua_assert( g->gcstate == GCSinsideatomic ||
                th->openupval == NULL || isintwups( th ) );

    for ( ; o < th->top; o++ ) { /* mark live elements in the stack */
        markvalue( g, o );
    }

    if ( g->gcstate == GCSinsideatomic ) { /* final traversal? */
        StkId lim = th->stack + th->stacksize;  /* real end of stack */

        for ( ; o < lim; o++ ) { /* clear not-marked stack slice */
            setnilvalue( o );
        }

        /* 'remarkupvals' may have removed thread from 'twups' list */
        if ( !isintwups( th ) && th->openupval != NULL ) {
            th->twups = g->twups;  /* link it back to the list */
            g->twups = th;
        }
    }
    else if ( g->gckind != KGC_EMERGENCY ) {
        luaD_shrinkstack( th );    /* do not change stack in emergency cycle */
    }

    return ( sizeof( lua_State ) + sizeof( TValue ) * th->stacksize +
             sizeof( CallInfo ) * th->nci );
}


/*
** traverse one gray object, turning it to black (except for threads,
** which are always gray).
*/
static void propagatemark( global_State* g ) {
    lu_mem size;
    GCObject* o = g->gray;
    lua_assert( isgray( o ) );
    gray2black( o );

    switch ( o->tt ) {
    case LUA_TTABLE: {
        Table* h = gco2t( o );
        g->gray = h->gclist;  /* remove from 'gray' list */
        size = traversetable( g, h );
        break;
    }

    case LUA_TLCL: {
        LClosure* cl = gco2lcl( o );
        g->gray = cl->gclist;  /* remove from 'gray' list */
        size = traverseLclosure( g, cl );
        break;
    }

    case LUA_TCCL: {
        CClosure* cl = gco2ccl( o );
        g->gray = cl->gclist;  /* remove from 'gray' list */
        size = traverseCclosure( g, cl );
        break;
    }

    case LUA_TTHREAD: {
        lua_State* th = gco2th( o );
        g->gray = th->gclist;  /* remove from 'gray' list */
        linkgclist( th, g->grayagain ); /* insert into 'grayagain' list */
        black2gray( o );
        size = traversethread( g, th );
        break;
    }

    case LUA_TPROTO: {
        Proto* p = gco2p( o );
        g->gray = p->gclist;  /* remove from 'gray' list */
        size = traverseproto( g, p );
        break;
    }

    default:
        lua_assert( 0 );
        return;
    }

    g->GCmemtrav += size;
}


static void propagateall( global_State* g ) {
    while ( g->gray ) {
        propagatemark( g );
    }
}


static void convergeephemerons( global_State* g ) {
    int changed;

    do {
        GCObject* w;
        GCObject* next = g->ephemeron;  /* get ephemeron list */
        g->ephemeron = NULL;  /* tables may return to this list when traversed */
        changed = 0;

        while ( ( w = next ) != NULL ) {
            next = gco2t( w )->gclist;

            if ( traverseephemeron( g, gco2t( w ) ) ) { /* traverse marked some value? */
                propagateall( g ); /* propagate changes */
                changed = 1;  /* will have to revisit all ephemeron tables */
            }
        }
    }
    while ( changed );
}

/* }====================================================== */


/*
** {======================================================
** Sweep Functions
** =======================================================
*/


/*
** clear entries with unmarked keys from all weaktables in list 'l' up
** to element 'f'
*/
static void clearkeys( global_State* g, GCObject* l, GCObject* f ) {
    for ( ; l != f; l = gco2t( l )->gclist ) {
        Table* h = gco2t( l );
        Node* n, *limit = gnodelast( h );

        for ( n = gnode( h, 0 ); n < limit; n++ ) {
            if ( !ttisnil( gval( n ) ) && ( iscleared( g, gkey( n ) ) ) ) {
                setnilvalue( gval( n ) ); /* remove value ... */
                removeentry( n ); /* and remove entry from table */
            }
        }
    }
}


/*
** clear entries with unmarked values from all weaktables in list 'l' up
** to element 'f'
*/
static void clearvalues( global_State* g, GCObject* l, GCObject* f ) {
    for ( ; l != f; l = gco2t( l )->gclist ) {
        Table* h = gco2t( l );
        Node* n, *limit = gnodelast( h );
        unsigned int i;

        for ( i = 0; i < h->sizearray; i++ ) {
            TValue* o = &h->array[i];

            if ( iscleared( g, o ) ) { /* value was collected? */
                setnilvalue( o );    /* remove value */
            }
        }

        for ( n = gnode( h, 0 ); n < limit; n++ ) {
            if ( !ttisnil( gval( n ) ) && iscleared( g, gval( n ) ) ) {
                setnilvalue( gval( n ) ); /* remove value ... */
                removeentry( n ); /* and remove entry from table */
            }
        }
    }
}


void luaC_upvdeccount( lua_State* L, UpVal* uv ) {
    lua_assert( uv->refcount > 0 );
    uv->refcount--;

    if ( uv->refcount == 0 && !upisopen( uv ) ) {
        luaM_free( L, uv );
    }
}


static void freeLclosure( lua_State* L, LClosure* cl ) {
    int i;

    for ( i = 0; i < cl->nupvalues; i++ ) {
        UpVal* uv = cl->upvals[i];

        if ( uv ) {
            luaC_upvdeccount( L, uv );
        }
    }

    luaM_freemem( L, cl, sizeLclosure( cl->nupvalues ) );
}


static void freeobj( lua_State* L, GCObject* o ) {
    switch ( o->tt ) {
    case LUA_TPROTO:
        luaF_freeproto( L, gco2p( o ) );
        break;

    case LUA_TLCL: {
        freeLclosure( L, gco2lcl( o ) );
        break;
    }

    case LUA_TCCL: {
        luaM_freemem( L, o, sizeCclosure( gco2ccl( o )->nupvalues ) );
        break;
    }

    case LUA_TTABLE:
        luaH_free( L, gco2t( o ) );
        break;

    case LUA_TTHREAD:
        luaE_freethread( L, gco2th( o ) );
        break;

    case LUA_TUSERDATA:
        luaM_freemem( L, o, sizeudata( gco2u( o ) ) );
        break;

    case LUA_TSHRSTR:
        luaS_remove( L, gco2ts( o ) ); /* remove it from hash table */
        luaM_freemem( L, o, sizelstring( gco2ts( o )->shrlen ) );
        break;

    case LUA_TLNGSTR: {
        luaM_freemem( L, o, sizelstring( gco2ts( o )->u.lnglen ) );
        break;
    }

    default:
        lua_assert( 0 );
    }
}


#define sweepwholelist(L,p) sweeplist(L,p,MAX_LUMEM)
static GCObject** sweeplist( lua_State* L, GCObject** p, lu_mem count );


/*
** sweep at most 'count' elements from a list of GCObjects erasing dead
** objects, where a dead object is one marked with the old (non current)
** white; change all non-dead objects back to white, preparing for next
** collection cycle. Return where to continue the traversal or NULL if
** list is finished.
*/
static GCObject** sweeplist( lua_State* L, GCObject** p, lu_mem count ) {
    global_State* g = G( L );
    int ow = otherwhite( g );
    int white = luaC_white( g ); /* current white */

    while ( *p != NULL && count-- > 0 ) {
        GCObject* curr = *p;
        int marked = curr->marked;

        if ( isdeadm( ow, marked ) ) { /* is 'curr' dead? */
            *p = curr->next;  /* remove 'curr' from list */
            freeobj( L, curr ); /* erase 'curr' */
        }
        else {   /* change mark to 'white' */
            curr->marked = cast_byte( ( marked & maskcolors ) | white );
            p = &curr->next;  /* go to next element */
        }
    }

    return ( *p == NULL ) ? NULL : p;
}


/*
** sweep a list until a live object (or end of list)
*/
static GCObject** sweeptolive( lua_State* L, GCObject** p ) {
    GCObject** old = p;

    do {
        p = sweeplist( L, p, 1 );
    }
    while ( p == old );

    return p;
}

/* }====================================================== */


/*
** {======================================================
** Finalization
** =======================================================
*/

/*
** If possible, shrink string table
*/
static void checkSizes( lua_State* L, global_State* g ) {
    if ( g->gckind != KGC_EMERGENCY ) {
        l_mem olddebt = g->GCdebt;

        if ( g->strt.nuse < g->strt.size / 4 ) { /* string table too big? */
            luaS_resize( L, g->strt.size / 2 );    /* shrink it a little */
        }

        g->GCestimate += g->GCdebt - olddebt;  /* update estimate */
    }
}


static GCObject* udata2finalize( global_State* g ) {
    GCObject* o = g->tobefnz;  /* get first element */
    lua_assert( tofinalize( o ) );
    g->tobefnz = o->next;  /* remove it from 'tobefnz' list */
    o->next = g->allgc;  /* return it to 'allgc' list */
    g->allgc = o;
    resetbit( o->marked, FINALIZEDBIT ); /* object is "normal" again */

    if ( issweepphase( g ) ) {
        makewhite( g, o );    /* "sweep" object */
    }

    return o;
}


static void dothecall( lua_State* L, void* ud ) {
    UNUSED( ud );
    luaD_callnoyield( L, L->top - 2, 0 );
}


static void GCTM( lua_State* L, int propagateerrors ) {
    global_State* g = G( L );
    const TValue* tm;
    TValue v;
    setgcovalue( L, &v, udata2finalize( g ) );
    tm = luaT_gettmbyobj( L, &v, TM_GC );

    if ( tm != NULL && ttisfunction( tm ) ) { /* is there a finalizer? */
        int status;
        lu_byte oldah = L->allowhook;
        int running  = g->gcrunning;
        L->allowhook = 0;  /* stop debug hooks during GC metamethod */
        g->gcrunning = 0;  /* avoid GC steps */
        setobj2s( L, L->top, tm ); /* push finalizer... */
        setobj2s( L, L->top + 1, &v ); /* ... and its argument */
        L->top += 2;  /* and (next line) call the finalizer */
        L->ci->callstatus |= CIST_FIN;  /* will run a finalizer */
        status = luaD_pcall( L, dothecall, NULL, savestack( L, L->top - 2 ), 0 );
        L->ci->callstatus &= ~CIST_FIN;  /* not running a finalizer anymore */
        L->allowhook = oldah;  /* restore hooks */
        g->gcrunning = running;  /* restore state */

        if ( status != LUA_OK && propagateerrors ) { /* error while running __gc? */
            if ( status == LUA_ERRRUN ) { /* is there an error object? */
                const char* msg = ( ttisstring( L->top - 1 ) )
                                  ? svalue( L->top - 1 )
                                  : "no message";
                luaO_pushfstring( L, "error in __gc metamethod (%s)", msg );
                status = LUA_ERRGCMM;  /* error in __gc metamethod */
            }

            luaD_throw( L, status ); /* re-throw error */
        }
    }
}


/*
** call a few (up to 'g->gcfinnum') finalizers
*/
static int runafewfinalizers( lua_State* L ) {
    global_State* g = G( L );
    unsigned int i;
    lua_assert( !g->tobefnz || g->gcfinnum > 0 );

    for ( i = 0; g->tobefnz && i < g->gcfinnum; i++ ) {
        GCTM( L, 1 );    /* call one finalizer */
    }

    g->gcfinnum = ( !g->tobefnz ) ? 0 /* nothing more to finalize? */
                  : g->gcfinnum * 2;  /* else call a few more next time */
    return i;
}


/*
** call all pending finalizers
*/
static void callallpendingfinalizers( lua_State* L ) {
    global_State* g = G( L );

    while ( g->tobefnz ) {
        GCTM( L, 0 );
    }
}


/*
** find last 'next' field in list 'p' list (to add elements in its end)
*/
static GCObject** findlast( GCObject** p ) {
    while ( *p != NULL ) {
        p = &( *p )->next;
    }

    return p;
}


/*
** move all unreachable objects (or 'all' objects) that need
** finalization from list 'finobj' to list 'tobefnz' (to be finalized)
*/
static void separatetobefnz( global_State* g, int all ) {
    GCObject* curr;
    GCObject** p = &g->finobj;
    GCObject** lastnext = findlast( &g->tobefnz );

    while ( ( curr = *p ) != NULL ) { /* traverse all finalizable objects */
        lua_assert( tofinalize( curr ) );

        if ( !( iswhite( curr ) || all ) ) { /* not being collected? */
            p = &curr->next;    /* don't bother with it */
        }
        else {
            *p = curr->next;  /* remove 'curr' from 'finobj' list */
            curr->next = *lastnext;  /* link at the end of 'tobefnz' list */
            *lastnext = curr;
            lastnext = &curr->next;
        }
    }
}


/*
** if object 'o' has a finalizer, remove it from 'allgc' list (must
** search the list to find it) and link it in 'finobj' list.
*/
void luaC_checkfinalizer( lua_State* L, GCObject* o, Table* mt ) {
    global_State* g = G( L );

    if ( tofinalize( o ) ||              /* obj. is already marked... */
            gfasttm( g, mt, TM_GC ) == NULL ) { /* or has no finalizer? */
        return;    /* nothing to be done */
    }
    else {  /* move 'o' to 'finobj' list */
        GCObject** p;

        if ( issweepphase( g ) ) {
            makewhite( g, o ); /* "sweep" object 'o' */

            if ( g->sweepgc == &o->next ) { /* should not remove 'sweepgc' object */
                g->sweepgc = sweeptolive( L, g->sweepgc );    /* change 'sweepgc' */
            }
        }

        /* search for pointer pointing to 'o' */
        for ( p = &g->allgc; *p != o; p = &( *p )->next ) { /* empty */ }

        *p = o->next;  /* remove 'o' from 'allgc' list */
        o->next = g->finobj;  /* link it in 'finobj' list */
        g->finobj = o;
        l_setbit( o->marked, FINALIZEDBIT ); /* mark it as such */
    }
}

/* }====================================================== */



/*
** {======================================================
** GC control
** =======================================================
*/


/*
** Set a reasonable "time" to wait before starting a new GC cycle; cycle
** will start when memory use hits threshold. (Division by 'estimate'
** should be OK: it cannot be zero (because Lua cannot even start with
** less than PAUSEADJ bytes).
*/
static void setpause( global_State* g ) {
    l_mem threshold, debt;
    l_mem estimate = g->GCestimate / PAUSEADJ;  /* adjust 'estimate' */
    lua_assert( estimate > 0 );
    threshold = ( g->gcpause < MAX_LMEM / estimate ) /* overflow? */
                ? estimate * g->gcpause  /* no overflow */
                : MAX_LMEM;  /* overflow; truncate to maximum */
    debt = gettotalbytes( g ) - threshold;
    luaE_setdebt( g, debt );
}


/*
** Enter first sweep phase.
** The call to 'sweeplist' tries to make pointer point to an object
** inside the list (instead of to the header), so that the real sweep do
** not need to skip objects created between "now" and the start of the
** real sweep.
*/
static void entersweep( lua_State* L ) {
    global_State* g = G( L );
    g->gcstate = GCSswpallgc;
    lua_assert( g->sweepgc == NULL );
    g->sweepgc = sweeplist( L, &g->allgc, 1 );
}


void luaC_freeallobjects( lua_State* L ) {
    global_State* g = G( L );
    separatetobefnz( g, 1 ); /* separate all objects with finalizers */
    lua_assert( g->finobj == NULL );
    callallpendingfinalizers( L );
    lua_assert( g->tobefnz == NULL );
    g->currentwhite = WHITEBITS; /* this "white" makes all objects look dead */
    g->gckind = KGC_NORMAL;
    sweepwholelist( L, &g->finobj );
    sweepwholelist( L, &g->allgc );
    sweepwholelist( L, &g->fixedgc ); /* collect fixed objects */
    lua_assert( g->strt.nuse == 0 );
}


static l_mem atomic( lua_State* L ) {
    global_State* g = G( L );
    l_mem work;
    GCObject* origweak, *origall;
    GCObject* grayagain = g->grayagain;  /* save original list */
    lua_assert( g->ephemeron == NULL && g->weak == NULL );
    lua_assert( !iswhite( g->mainthread ) );
    g->gcstate = GCSinsideatomic;
    g->GCmemtrav = 0;  /* start counting work */
    markobject( g, L ); /* mark running thread */
    /* registry and global metatables may be changed by API */
    markvalue( g, &g->l_registry );
    markmt( g ); /* mark global metatables */
    /* remark occasional upvalues of (maybe) dead threads */
    remarkupvals( g );
    propagateall( g ); /* propagate changes */
    work = g->GCmemtrav;  /* stop counting (do not recount 'grayagain') */
    g->gray = grayagain;
    propagateall( g ); /* traverse 'grayagain' list */
    g->GCmemtrav = 0;  /* restart counting */
    convergeephemerons( g );
    /* at this point, all strongly accessible objects are marked. */
    /* Clear values from weak tables, before checking finalizers */
    clearvalues( g, g->weak, NULL );
    clearvalues( g, g->allweak, NULL );
    origweak = g->weak;
    origall = g->allweak;
    work += g->GCmemtrav;  /* stop counting (objects being finalized) */
    separatetobefnz( g, 0 ); /* separate objects to be finalized */
    g->gcfinnum = 1;  /* there may be objects to be finalized */
    markbeingfnz( g ); /* mark objects that will be finalized */
    propagateall( g ); /* remark, to propagate 'resurrection' */
    g->GCmemtrav = 0;  /* restart counting */
    convergeephemerons( g );
    /* at this point, all resurrected objects are marked. */
    /* remove dead objects from weak tables */
    clearkeys( g, g->ephemeron, NULL ); /* clear keys from all ephemeron tables */
    clearkeys( g, g->allweak, NULL ); /* clear keys from all 'allweak' tables */
    /* clear values from resurrected weak tables */
    clearvalues( g, g->weak, origweak );
    clearvalues( g, g->allweak, origall );
    luaS_clearcache( g );
    g->currentwhite = cast_byte( otherwhite( g ) ); /* flip current white */
    work += g->GCmemtrav;  /* complete counting */
    return work;  /* estimate of memory marked by 'atomic' */
}


static lu_mem sweepstep( lua_State* L, global_State* g,
                         int nextstate, GCObject** nextlist ) {
    if ( g->sweepgc ) {
        l_mem olddebt = g->GCdebt;
        g->sweepgc = sweeplist( L, g->sweepgc, GCSWEEPMAX );
        g->GCestimate += g->GCdebt - olddebt;  /* update estimate */

        if ( g->sweepgc ) { /* is there still something to sweep? */
            return ( GCSWEEPMAX * GCSWEEPCOST );
        }
    }

    /* else enter next state */
    g->gcstate = nextstate;
    g->sweepgc = nextlist;
    return 0;
}


static lu_mem singlestep( lua_State* L ) {
    global_State* g = G( L );

    switch ( g->gcstate ) {
    case GCSpause: {
        g->GCmemtrav = g->strt.size * sizeof( GCObject* );
        restartcollection( g );
        g->gcstate = GCSpropagate;
        return g->GCmemtrav;
    }

    case GCSpropagate: {
        g->GCmemtrav = 0;
        lua_assert( g->gray );
        propagatemark( g );

        if ( g->gray == NULL ) { /* no more gray objects? */
            g->gcstate = GCSatomic;    /* finish propagate phase */
        }

        return g->GCmemtrav;  /* memory traversed in this step */
    }

    case GCSatomic: {
        lu_mem work;
        propagateall( g ); /* make sure gray list is empty */
        work = atomic( L ); /* work is what was traversed by 'atomic' */
        entersweep( L );
        g->GCestimate = gettotalbytes( g );  /* first estimate */;
        return work;
    }

    case GCSswpallgc: {  /* sweep "regular" objects */
        return sweepstep( L, g, GCSswpfinobj, &g->finobj );
    }

    case GCSswpfinobj: {  /* sweep objects with finalizers */
        return sweepstep( L, g, GCSswptobefnz, &g->tobefnz );
    }

    case GCSswptobefnz: {  /* sweep objects to be finalized */
        return sweepstep( L, g, GCSswpend, NULL );
    }

    case GCSswpend: {  /* finish sweeps */
        makewhite( g, g->mainthread ); /* sweep main thread */
        checkSizes( L, g );
        g->gcstate = GCScallfin;
        return 0;
    }

    case GCScallfin: {  /* call remaining finalizers */
        if ( g->tobefnz && g->gckind != KGC_EMERGENCY ) {
            int n = runafewfinalizers( L );
            return ( n * GCFINALIZECOST );
        }
        else {   /* emergency mode or no more finalizers */
            g->gcstate = GCSpause;  /* finish collection */
            return 0;
        }
    }

    default:
        lua_assert( 0 );
        return 0;
    }
}


/*
** advances the garbage collector until it reaches a state allowed
** by 'statemask'
*/
void luaC_runtilstate( lua_State* L, int statesmask ) {
    global_State* g = G( L );

    while ( !testbit( statesmask, g->gcstate ) ) {
        singlestep( L );
    }
}


/*
** get GC debt and convert it from Kb to 'work units' (avoid zero debt
** and overflows)
*/
static l_mem getdebt( global_State* g ) {
    l_mem debt = g->GCdebt;
    int stepmul = g->gcstepmul;

    if ( debt <= 0 ) {
        return 0;    /* minimal debt */
    }
    else {
        debt = ( debt / STEPMULADJ ) + 1;
        debt = ( debt < MAX_LMEM / stepmul ) ? debt * stepmul : MAX_LMEM;
        return debt;
    }
}

/*
** performs a basic GC step when collector is running
*/
void luaC_step( lua_State* L ) {
    global_State* g = G( L );
    l_mem debt = getdebt( g ); /* GC deficit (be paid now) */

    if ( !g->gcrunning ) { /* not running? */
        luaE_setdebt( g, -GCSTEPSIZE * 10 ); /* avoid being called too often */
        return;
    }

    do {  /* repeat until pause or enough "credit" (negative debt) */
        lu_mem work = singlestep( L ); /* perform one single step */
        debt -= work;
    }
    while ( debt > -GCSTEPSIZE && g->gcstate != GCSpause );

    if ( g->gcstate == GCSpause ) {
        setpause( g );    /* pause until next cycle */
    }
    else {
        debt = ( debt / g->gcstepmul ) * STEPMULADJ; /* convert 'work units' to Kb */
        luaE_setdebt( g, debt );
        runafewfinalizers( L );
    }
}


/*
** Performs a full GC cycle; if 'isemergency', set a flag to avoid
** some operations which could change the interpreter state in some
** unexpected ways (running finalizers and shrinking some structures).
** Before running the collection, check 'keepinvariant'; if it is true,
** there may be some objects marked as black, so the collector has
** to sweep all objects to turn them back to white (as white has not
** changed, nothing will be collected).
*/
void luaC_fullgc( lua_State* L, int isemergency ) {
    global_State* g = G( L );
    lua_assert( g->gckind == KGC_NORMAL );

    if ( isemergency ) {
        g->gckind = KGC_EMERGENCY;    /* set flag */
    }

    if ( keepinvariant( g ) ) { /* black objects? */
        entersweep( L ); /* sweep everything to turn them back to white */
    }

    /* finish any pending sweep phase to start a new cycle */
    luaC_runtilstate( L, bitmask( GCSpause ) );
    luaC_runtilstate( L, ~bitmask( GCSpause ) ); /* start new collection */
    luaC_runtilstate( L, bitmask( GCScallfin ) ); /* run up to finalizers */
    /* estimate must be correct after a full GC cycle */
    lua_assert( g->GCestimate == gettotalbytes( g ) );
    luaC_runtilstate( L, bitmask( GCSpause ) ); /* finish collection */
    g->gckind = KGC_NORMAL;
    setpause( g );
}

/* }====================================================== */


